1. Field of the Invention
The present invention relates to LED writers and more particularly to method for correcting for non-uniformities associated with such writers.
2. Description Relative to the Prior Art
With reference to PCT publication No. WO 91/10311, the pertinent contents of which are incorporated herein by reference, in an LED-based gray scale electrophotographic printing system, exposure is usually controlled by the length of time each LED is turned on with the emitted light intensity held constant. The LED on-time varies imagewise and the resulting exposure is given by the product of this time and the intensity. The printhead itself is comprised of several thousand individual LEDs and, because of variation in LED response, the light intensity varies from LED to LED within the same printhead for the same amount of energizing current. This results in a non-uniform exposure of an otherwise uniform field.
For each level of gray the number of exposure times that can be requested is potentially equal to the number of LEDs in the printhead. Thus, for a j-bit gray level system that uses an LED printhead with N LEDs as the writer, the number of possible exposures that can be requested is N (2.sup.j- 1), excluding the null exposure level (white). For N=4000 (typical) and j=2, this number is 12,000. There is no economical printhead architecture that is capable of generating nearly as many exposures as this. The number of exposures that a typical LED printhead can generate is determined by its controller circuit. For a printhead with a k-bit controller, the total number of exposures that can be generated is 2.sup.k, with j.ltoreq.k but significantly less than N (2.sup.j- 1). It is the goal of a non-uniformity correction algorithm to use the total number of exposures that can be requested to generate an optimum look-up table (LUT) with 2.sup.k entries. The procedure for generating the LUT must condense in some optimal manner all the exposures that can be requested (including zero) into 2.sup.k exposures.
The actual number of exposures that can be requested depends on the distribution of the LED intensifies and the range and distribution of the (2.sup.j- 1) gray levels. This number is usually less than the number quoted above because some of the exposures coincide.
In FIG. 1, there is shown the distribution of LED intensities for a k=6-bits per pixel controller printhead. This printhead has 4608 LEDs of which several (at the extreme ends) are dead. If this printhead is used to print 4-bit gray levels, the total number of distinct exposures must be available for perfect uniformity is somewhat less than 2000, depending on the 15 gray levels selected. This number of available exposures while quite large is still smaller than it could otherwise be because of overlap in the gray levels. For the 15 gray levels shown in Table 1 below, the distribution of exposure times is shown in FIG. 2. FIGS. 3a and 3b show the kind of uniformity (as print and power spectrum, respectively) obtained from the uncorrected exposures. Note in all the uniformity prints in this invention, only the last 10 steps are shown. The remaining 5 steps are too light to be reproduced faithfully. The power spectrum plots are for the eighth of the fifteen steps; i.e., the third lightest of those shown here.) The huge spike in FIG. 3b is due to the imbedded non-uniformity of the Selfoc lenses covering the LEDs. The print in FIG. 3a and the plot in FIG. 3b clearly demonstrates the need to correct the non-uniformity so that each gray level or step of a flat field is rendered as a uniform field.
TABLE 1 ______________________________________ Exposure Time Step # (microseconds) ______________________________________ 1 4.70 2 11.01 3 12.74 4 14.27 5 15.76 6 17.02 7 18.42 8 20.10 9 21.84 10 23.76 11 25.90 12 28.51 13 31.59 14 35.52 15 44.68 ______________________________________
Current methods of correcting this non-uniformity fall into two broad categories (1) linear quantization of the minimum-maximum range of exposures that can be requested into the number of available exposures; i.e., 2.sup.k levels and (2) nonlinear quantization of same range into same number of levels. These two methods have been described and practiced in U.S. Pat. No. 4,982,203 to set the initial corrected exposure for each LED to achieve uniformity. These settings are subsequently adjusted to compensate for the effects of aging and thermal heating of the LEDs. Another type of nonlinear quantization is employed in U.S. Pat. No. 4,857,944, both for the initial calibration and subsequent recalibrations. The main drawbacks of this kind of nonlinear quantization are 1) it addresses only the binary. (i.e. j=1) case and 2) it is not easy to implement.
Another kind of nonlinear quantization method is that described in U.S. Pat. No. 5,200,765. In this method, the total number of available printhead levels (2.sup.k- 1) is divided nonlinearly among the 2.sup.j -1 input gray levels (white excluded). The exposure range for each gray step is then quantized linearly into the number of levels assigned to it. The more levels a gray step receives, the more uniform that level is rendered. More levels are assigned to the light areas than to the darker ones because the eye is most sensitive to noise in the low density region. If the levels are assigned properly, this method can result in very good uniformity across the density spectrum. The main drawbacks of this technique are 1) the correct level assignment to achieve a particular uniformity is done on a trial and error basis, thereby requiring the running of the algorithm several times and user intervention each time to enter the levels, 2) the algorithm is not as robust as the method of my invention with respect to the gray levels selected and the LED intensity distribution, and 3) the procedure may not be optimal because of the linear quantization used for each gray level.
There is presently a need for an easy-to-implement-and-use process that corrects for the inherent non-uniformity of LED printhead. The needed algorithm must be optimally robust with respect to the selected gray levels and LED intensity distribution and must be capable of achieving specified or target uniformities for any gray level or group of gray levels.